This invention relates to a process for producing liquid and, optionally, gaseous products from gaseous reactants.
According to a first aspect of the invention, there is provided a process for producing liquid and, optionally, gaseous products from gaseous reactants, which process comprises
feeding, at a low level, gaseous reactants into a slurry bed of solid catalyst particles suspended in a suspension liquid;
allowing the gaseous reactants to react as they pass upwardly through the slurry bed, thereby to form liquid and, optionally, gaseous products, with the reaction being catalyzed by the catalyst particles; and
separating liquid product from the catalyst particles by passing, in a filtration zone within the slurry bed, liquid product through a filtering medium having a plurality of openings through which the liquid passes, with the openings having a controlling dimension of x microns, and with the proportion of catalyst particles, which have a particle size smaller than x microns, in the slurry bed being less than 18% by volume based on the total volume of the catalyst in the slurry bed.
A low proportion, or even absence, of such fine catalyst particles in the slurry bed ensures a high solids separation efficiency across the filtering medium, and also ensures a low degree of catalyst contamination of the liquid product downstream of the filtering medium; however, as also described in more detail hereunder, it has surprisingly been found that high conversions of gaseous reactants to products are nevertheless obtainable in the process. Thus, a low proportion of less than 4 vol %, preferably less than 2 vol %, catalyst particles smaller than x microns, normally ensures that the liquid product has a catalyst content less than 10 ppm (by mass)
By xe2x80x98controlling dimensionxe2x80x99 in respect of the filtering medium openings or filtering openings is meant the maximum dimension of the filtering openings through which the catalyst particles can pass. The controlling dimension may, for example, be obtained from the filter manufacturer""s specification. Thus, it may be the upper tolerance level, or it may be the average gap size added to a factor of, e.g. three times, the gap size standard deviation.
The filtering openings may thus be of any desired shape. In one embodiment, the filtering medium openings, when seen in the direction of liquid flow through the openings, may be circular, with the controlling dimension of each opening thus being its diameter. Instead, in another embodiment, the filtering medium openings, when seen in the direction of liquid flow through the openings, may be more-or-less rectangular so that the width of each opening is shorter than its length, with the controlling dimension of each opening being its width.
Thus, for example, x may typically be 40 microns. The filtering medium may then, for example, be a wedge wire filtering medium comprising parallel wires which are spaced so as to provide openings whose average widths are 10 microns, i.e. the filtering medium has a nominal opening or gap size of 10 microns; the opening widths thus vary or deviate from 10 microns, with the average width being 10 microns, while the maximum width or gap size is 40 microns. The slurry bed will then contain less than 18% by volume (based on the volume of the total catalyst inventory of the bed) of catalyst particles having a diameter less than 40 microns, ie particles smaller than 40 microns.
The proportion of catalyst particles smaller than x microns in the slurry bed may thus be less than 18% by volume at the start of a catalyst run, ie when the catalyst is initially loaded in a slurry phase reactor or on initial formation of the slurry bed at the start of the ruin. However, during the course of the run, the proportion of catalyst particles smaller than x microns is worked down, through normal operation of the slurry bed, to less than 4 vol %, and preferably less than 2 vol %. Accordingly, the slurry bed may typically contain substantially no catalyst particles smaller than x microns for at least a major portion of the catalyst run, eg for substantially the entire run.
In a particular embodiment of the invention, the controlling dimension of the filtering medium openings may be their minimum dimension, with the proportion of catalyst particles, whose minimum dimension is less than x microns, in the slurry bed being less than 4% by volume based on the total volume of the catalyst in the slurry bed.
While the process can, at least in principle, have broader application, it is envisaged that the suspension liquid will normally, but thus not necessarily always, be the liquid product.
Furthermore, while it is also believed that, in principle, the process can have broader application, it is envisaged that it will have particular application in hydrocarbon synthesis where the gaseous reactants are capable of reacting catalytically in the slurry bed to form liquid hydrocarbon product and, optionally, gaseous hydrocarbon product. In particular, the hydrocarbon synthesis may be Fischer-Tropsch synthesis, with the gaseous reactants being in the form of a synthesis gas stream comprising mainly carbon monoxide and hydrogen, with both liquid and gaseous hydrocarbon products being produced, and with the catalyst particles thus being Fischer-Tropsch catalyst particles.
The slurry bed will thus be provided in a suitable vessel, eg a column, with unreacted reactants and gaseous product being withdrawn from the vessel above the slurry bed, and the separated liquid product also being withdrawn from the vessel. The vessel will be maintained at normal elevated pressure and temperature conditions associated with Fischer-Tropsch synthesis, eg a predetermined operating pressure in the range 10 to 50 bar, and at a predetermined temperature in the range 180xc2x0 C. and 280xc2x0 C., or even higher for the production of lower boiling point product.
Any suitable filtering medium can, at least in principle, be used, and the filtering medium may have differing opening or gap sizes. However, all the openings of the filtering medium will normally be of nominally the same size and have the same geometry. The filtering medium may be part of a filter cartridge or element mounted in the vessel, and may be of a type which is of elongate form, with the filtering medium being of cylindrical form and enclosing a filtrate collecting zone, and with a filtrate outlet for withdrawing filtrate, ie liquid product, being provided at one end thereof. While, in principle, the filtering medium can be any desired filtering medium having the desired opening size to prevent catalyst particles passing therethrough, it is preferably of a type or construction with which permanent clogging or impregnation thereof with the catalyst particles does not readily occur. Thus, the filtering medium can be a mesh, eg a woven mesh; a porous material such as a ceramic material; a perforated sheet; spiral wire wound, eg from wedge wire; or the like.
The maximum allowable controlling dimension of the filtering medium will thus be dictated by the portion of catalyst particle sizes smaller than the controlling dimension of the filter, present in the slurry bed. Although, in slurry phase reactions, catalyst breakup due to attrition normally takes place, resulting in a lowering of the minimum particle size, and a decrease in the average catalyst particle size, it has surprisingly been found that catalyst breakup by attrition or any other means of disintegration can be almost entirely avoided.
The catalyst particles can, at least in principle, be any desired supported Fischer-Tropsch catalyst, such as an iron-based catalyst, a cobalt-based catalyst, or any other Fischer-Tropsch catalyst. Supported catalysts, which are physically stronger than unsupported catalysts, are typically used, and supported cobalt catalysts are preferred. Preferably, the catalyst may be that obtained by a preparation method as described in ZA 96/2759//U.S Pat. No. 5,733,839 or ZA 99/1265//PCT/GB99/00527. Such a method is hereinafter also referred to as xe2x80x98the proprietary methodxe2x80x99, and the catalyst obtained is hereinafter also referred to as the xe2x80x98proprietary cobalt Fischer-Tropsch catalystxe2x80x99. ZA 96/2759//U.S. Pat. No. 5,733,839 and ZA 99/1265//PCT/GB99/00527, are hence incorporated herein by reference thereto. It was surprisingly found that catalysts produced in this manner are sufficiently strong so that little or no attrition thereof during extended runs at normal slurry bed operating conditions occurs. In other words, it was surprisingly found that, when the catalyst which is used in the process is prepared in accordance with the method of ZA 96/2759//U.S. Pat. No. 5,733,839 or ZA 99/1265//PCT/GB99/00527, the catalyst particle size distribution can be selected so that there are less than 5 vol % of particles smaller than 45 microns initially present; moreover, it was surprisingly found with these catalysts that the particle size distribution remains more-or-less unchanged during operation under normal operating conditions, ie there normally is little or no attrition into particles smaller than 45 microns.
When using catalyst particles that are physically strong, eg supported cobalt catalyst particles, it is important to ensure that the slurry bed at all times does not contain an appreciable proportion of catalyst particles of a size close to the aperture or opening size of the filtering medium, ie near gap-size particles, since such sized particles can cause permanent blockage of the filtering medium.
The process of the invention thus permits the selection of an optimized catalyst size distribution for a Fischer Tropsch catalyst based on a specific catalyst support, with the catalyst having sufficient strength and an appropriate size distribution to maximize synthesis performance and to ensure trouble free extended continuous operation of the reactor vessel. The production of sufficiently strong catalysts is, as set out hereinbefore, described in ZA 96/2759//U.S. Pat. No. 5,733,839 and ZA 99/1265//PCT/GB99/00527, and comprises mixing a modified catalyst support with an aqueous solution of an active catalyst component or its precursor, to form a slurry, and impregnating the protected modified catalyst support with the active catalyst component or its precursor, to form the catalyst. The method of modifying the catalyst support comprises introducing onto and/or into an untreated catalyst support which is partially soluble in an aqueous acid solution and/or a neutral aqueous solution, a modifying component capable, when present in or on the catalyst support, of suppressing the solubility of the catalyst support in the aqueous acid solution and/or the neutral aqueous solution, thereby to form a modified catalyst support which is less soluble or more inert in the aqueous acid solution and/or the neutral aqueous solution, than the untreated catalyst support. The modifying component may comprise foreign atoms such as Cu, Zn, Mn, Ba, Si, Co, Ni, Zr, Ce, or Mg.
The Applicant has surprisingly found that by selecting a catalyst support material with sufficient strength and an appropriate size distribution, the optimized catalyst particle size distribution of a 30 g Co/100 g Al2O3 catalyst prepared on a pre-shaped Al2O3 support with a BET surface area of 150 m2/g and a BET porosity of 0.50 ml/g, according to the proprietary method, is 45 to 250 microns, and preferably 70 to 200 microns. Provided the bulk, ie at least 90% of the particles, are within the preferred size range of 70 to 200 microns after losing the portion of material smaller than the controlling dimension of the filter, the desired synthesis performance will essentially be attained. A further advantage obtained with the selection of a catalyst support material with sufficient strength and an appropriate size distribution is that the formation of fines during Fischer-Tropsch synthesis in the slurry phase reactor is largely prevented, thereby normally ensuring a liquid product that has a catalyst content less than 10 ppm (by mass).
To ensure effective reaction and optimum production rates, interaction between the said particles and gas molecules is essential, and therefore catalyst suspension is important. It has hitherto been believed that it is imperative to have a substantial proportion of relatively small catalyst particles present in the slurry bed. Particularly, in the absence of downcomers, with which the solid particulate catalyst phase can be kept in near uniform suspension by using gas bubbles, a substantial proportion of catalyst particles of the order of 30 microns have hitherto been believed to be essential.
To inhibit catalyst settling, the process may thus include agitating the slurry in the slurry bed. The agitation may include allowing slurry in the slurry bed to pass downwardly from a high level to a lower level, through at least one downcomer. Preferably, the slurry may be allowed to pass downwardly through at least one downcomer located in a first downcomer region of the slurry bed, as well as through at least one further downcomer located in a second downcomer region of the slurry bed, with the second downcomer region being spaced vertically with respect to the first downcomer region, so as to redistribute the catalyst particles within the slurry bed, as taught in ZA 98/5992 // PCT/GB98/02070 which is hence incorporated herein by reference. Thus, the downcomer(s) serve(s) to impart a nett upward liquid velocity to the slurry bed in the regions of the slurry bed outside the downcomers thereby maintaining the catalyst in near uniform suspension.
The process thus involves initially selecting a catalyst particle size distribution in a specific size range so that 18 vol %, preferably 4 vol %, more preferably 2 vol %, of the particle sizes are less than the controlling dimension of the filtering medium, thereby avoiding, or at least reducing, permanent or irreversible blinding of the filtering medium, and catalyst loss into the liquid product. The maximum particle size, which in turn determines the range of particle sizes, is then determined to avoid lower activity or poor selectivity due to intra-particle mass transfer effects. This is achieved by following known procedures, eg as illustrated in Example 4. It is preferable that more than 90% of the catalyst particles are less than the maximum particle size. Additionally, there is little or no loss of catalyst contact with the gaseous reactants due to catalyst particle settling.
The process may include operating the column such that the slurry bed is in a heterogeneous or churn-turbulent flow regime and comprises a dilute phase consisting of fast-rising large bubbles of gaseous reactants, and possibly gaseous products which traverse the reaction zone or slurry bed virtually in a plug flow manner and a dense phase comprising liquid phase, ie liquid product, solid catalyst particles and entrained smaller bubbles of gaseous reactants and gaseous product.
By passing or recirculating some of the slurry through the downcomers, more uniform redistribution of the catalyst in the slurry bed is achieved, than is the case without such downcomers. The catalyst particles in the slurry bed are thus maintained in suspension by the turbulence created by the synthesis gas stream passing through the slurry bed combined with an upward liquid velocity induced by the presence of the downcomers. It was found that the use of downcomers to keep the catalyst particles in uniform suspension, avoids the problem of catalyst settling when selecting the optimum catalyst particle size distribution. Computational fluid dynamic (CFD) modelling may be used to optimize the layout of the downcomers.
The process may thus include allowing a cake of catalyst particles to form on the filtration medium; from time to time interrupting the passage of liquid product through the filtering medium; and backflushing the filtering medium in the opposite direction to the direction of flow through the filtering medium during the separation of the liquid product from the catalyst particles, thereby to dislodge the cake from the filtering medium. The backflushing may be effected for at least portions of the periods that the liquid product passage through the filtering medium is interrupted.
A plurality of the filter elements, located at the same or different levels within the filtration zone, may be provided. The filtration zone may be provided anywhere below the upper surface of the slurry bed. The filter elements may be arranged in a plurality of banks, with each filter bank comprising a number of the-filter elements.
In principle, the elements can be located at any desired inclination; however, they are preferably located vertically with their liquid product or filtrate outlets directed downwardly.
The passage of the liquid product through the filtering media may be effected by applying a pressure differential across the filtering media and any cake build-up thereon. Preferably this pressure differential may be up to 8 bar, and is typically in the region of between 1 and 4 bar. The pressure differential may be effected by withdrawing the liquid product into a rundown vessel which is at a lower pressure than the reactor vessel, with the filtrate outlets of the filter elements being connected to the rundown vessel by means of suitable liquid product conduits. The conduits may include a primary liquid product conduit leading from the filtrate outlet of each filter element; a secondary liquid product conduit into which the primary conduits of all the filter elements of the particular bank of filter elements tie; and a tertiary liquid product conduit leading to the rundown vessel, with the secondary conduits all tying into the tertiary conduit.
The flushing fluid may be process or non-process derived liquid and/or gas, eg some of the liquid and/or gaseous product.
The backflushing may, in general, be effected in pulse-like fashion. Thus, the backflushing may comprise an initial pulse of flushing liquid and/or gas, optimally followed by one or more further pulses of flushing liquid and/or gas. Each backflushing pulse may comprise initiating backflushing rapidly, ie commencing flow of flushing fluid rapidly; and backflushing the elements rapidly with a volume of the flushing fluid. This volume of flushing fluid may be relatively large, eg approximately equivalent to the internal volume of the filter elements. It can, however, be less than the internal volume of the filter elements, eg less than half their internal volume. When the volume of flushing fluid used during the initial pulse is relatively large, the volume of flushing fluid used during a second pulse may be less than that of the initial pulse, eg less than half the internal volume of the elements. However, when the volume of flushing fluid used during an initial pulse is relatively small as hereinbefore described, then the volume of flushing fluid during a further or second pulse may be similar to that of the initial pulse. The nature of any further pulses, when utilized, and the volume of the flushing fluid used during such pulses, may be similar to those of the second pulse hereinbefore described.
The pressure differential across the filtering media and filter cake during backflushing may be up to 10 bar depending on the degree of clogging or age of the filtering media, and is typically at least 1 bar higher than the filtration pressure differential.
The flushing fluid flow rate may be at least 6000 l/h/m2 of filtering media. Thus, the flushing fluid flow rate may be between 6000 l/h/m2 of filtering media when the pressure differential across the filtering media is about 5 bar, and between about 10000 and 12000 l/h/m2 when the pressure differential is about 10 bar.
The process may preferably include subjecting the filtering elements to a waiting period during which no filtering or backflushing takes place, ie during which there is no liquid flow through the filtering media of the elements, to enhance subsequent filtration. The waiting period may be up to 60 minutes, or even longer, but is typically less than 30 minutes.
The backflushing may thus be effected in a manner, and using backflushing means, as described in ZA 94/0582 // U.S. Pat. No. 5,599,849/U.S. Pat. No. 5,844,006, which is hence incorporated herein by reference. Thus, backflushing may be effected by propelling or forcing residual liquid product in the conduits back through the filter elements in the second direction, preferably also through a restriction orifice located in the primary conduit of each filter element, by means of pressurized gas. It has been found that cleaning of the filter element surfaces is considerably improved, thus enhancing subsequent filtering performance thereof, when backflushing with the gas is effected for at least a sufficiently long period of time to displace substantially all the residual liquid product through the restriction orifices back into the filter elements. Backflushing with gas also has the advantage that the gas thereby introduced into the reactor vessel and which is removed with the product gas, does not have to be filtered again, thereby reducing loading on the filter elements during filtration.
According to a second aspect of the invention, there is provided a process for producing liquid and, optionally, gaseous products from gaseous reactants, which process comprises
feeding, at a low level, gaseous reactants into a slurry bed of solid catalyst particles suspended in a suspension liquid;
allowing the gaseous reactants to react as they pass upwardly through the slurry bed, thereby to form liquid and, optionally, gaseous products, with the reaction being catalyzed by the catalyst particles; and
separating liquid product from the catalyst particles by passing, in a filtration zone within the slurry bed, liquid product through a filtering medium having a plurality of openings through which the liquid passes, with the openings having a minimum dimension of x microns, and with the proportion of catalyst particles, whose minimum dimension is less than x microns, in the slurry bed being less than 4% by volume based on the total volume of the catalyst in the slurry bed.
The invention will now be described by way of example with reference to the accompanying diagrammatic drawings.